You're not a monster. Quite the opposite. I'd say at this point it's more important for your babies to have a sane mommie than to continue BFing both. There's a reason you're feeling this way -- you're exhausted and burned out. So don't beat up on yourself. You've done an amazing job to have been BFing continuously since Feb. '07 and two children for a lot of that time! I am sure there are lots of LLLadies out there who can give specific advice about weaning one while feeding two.
Other than letting you know you've done an amazing job and you're not a monster I don't have any specific advice as I have one child and we are now trying to wean from the last bastion of BFing -- the nightie-night nurse.
Best of luck to you -- hope you have some at-home help getting where you want to be.